1. Field of the Invention
The present invention relates to microanalysis techniques for examining material microstructure in a scanning electron microscope or other suitable instrument. In particular, the present invention applies to electron backscatter diffraction in a scanning electron microscope or other suitable instrument to examine the crystallographic aspects of materials.
2. Description of the Prior Art
Electron backscatter diffraction (EBSD) is used by scientists and engineers who need to examine the crystallographic aspects of microstructure. This information can be particularly important in materials used in transportation applications, for example, in aircraft and automobiles. Factors such as crystal orientation and the nature of grain boundaries affect the mechanical and electrical properties of such materials and are, therefore, important design parameters. Unlike X-ray diffraction, EBSD provides scientists and engineers with direct measurements of local orientation that can be correlated with material properties.
Electron backscatter diffraction patterns (EBSP) are obtained in a scanning electron microscope (SEM) by focusing the electron beam on a crystalline sample. The sample is tilted to approximately 70 degrees with respect to the horizontal, and the diffraction pattern is imaged on a phosphor screen. The image is captured using a low-light charge coupled device (CCD) camera or a silicon-intensifier target (SIT) camera. The bands in the pattern represent the reflecting planes in the diffracting crystal volume. Thus, the geometrical arrangements of the bands are a function of the crystallographic orientation and symmetry of the diffraction crystal lattice.
Some success has been documented for differentiating phases by EBSD. However, when the crystallographic structure of two phases are similar it may not be possible to unambiguously distinguish one phase from another using only EBSD.
Energy dispersive spectroscopy (EDS) is a microanalytical technique based on the characteristic X-ray spectrum peaks that are generated when the high energy beam of an electron microscope interacts with a specimen. When a stationary beam of high voltage electrons is focused on a specimen, atoms in the specimen are placed in an excited state. When the excited atoms return to the ground state, they emit an X-ray of characteristic energy and wavelength. This characteristic energy and wavelength is a function of the difference in electron energy levels of the atom. Therefore, each element in a specimen produces an X-ray emission having a characteristic spectral fingerprint that may be used to identify the presence of that element within the specimen.
Some success has been achieved in differentiating phases by EDS. However, when the chemical composition of the two phases are similar it may not be possible to distinguish one phase from another using only EDS.
U.S. Pat. No. 5,266,802 to Kasai discloses an electron microscope having an objective lens and an EDS detector attached thereto.
U.S. Pat. No. 6,326,619 to Michael et al. discloses a method and apparatus for determining the crystalline phase and crystalline characteristics of a sample by using an electron beam generator, such as a scanning electron microscope, to obtain a backscattered electron Kikuchi pattern of a sample, and extracting crystallographic and composition data that is matched to database information to provide a quick and automatic method to identify crystalline phases.
It would, therefore, be desirable to be able to simultaneously collect crystallographic data and chemical data and combine them to obtain reliably differentiated phase data for a sample.
The present invention is directed to an analytical method for combining chemical information with crystallographic information to obtain a map of the crystal orientation, the nature of grain boundaries and distinguishing crystalline phases in a polycrystalline sample. The method broadly encompasses filtering crystallographic data using the chemical information to provide a map of the crystal orientation and grain boundaries of the sample including the following steps, in any suitable order, of:
providing a list of phases that may be present in a region of interest in a sample to include crystallographic structural parameters for each phase and upper and lower limits for the amount of each element that may be present in each of the listed phases;
identifying the elements present in the region of interest of the sample at a plurality of point locations;
obtaining an electron backscatter diffraction (EBSD) pattern at each of the plurality of point locations in the region of interest;
determining the location of and characteristics of the bands in the EBSD pattern (EBSP);
applying a chemical filter by comparing the amounts of each element at each point against the upper limits and lower limits for a given element with each of the phases in the list of phases to determine a set of possible phases for the point;
assigning a phase to each point by comparing the EBSD band locations and characteristics against the structure parameters for each of the possible phases and determining the best match; and
determining the crystallographic orientation of the phase at each of the plurality of point locations in the region of interest.
The present invention is further directed to an instrument capable of performing the above-described method. The instrument generally includes a scanning electron microscope having a means for applying a collimated electron beam to a sample, a means for obtaining an EBSP, and a means for determining the elemental composition of the sample, as well as a means for recording EBSD band locations and characteristics and the elemental composition of the sample.